1. Field of the Invention
The present invention relates to a turbo molecular pump, and in particular to a turbo molecular pump which can release heat generated by an impeller without cooling its base section.
2. Description of the Related Art
In case where aluminum chloride or the like is discharged from the turbo molecular pump, the inside of the turbo molecular pump may exceed the saturation vapor pressure where the solidification or adhesion of the produced material occurs. FIG. 6 shows an entire configuration of the turbo molecular pump. In FIG. 6, an impeller 2 is mounted on a rotational shaft 5 for rotation therewith. The impeller 2 is designed so as to rotate while being floated by a magnetic bearing. An electric component section is provided, which includes radial electromagnets 20, 21, radial position detectors 22, 23 and a motor 30 for rotationally driving the shaft 5. Especially at an impeller 2 and a screw-threaded spacer 4 near a discharge port 26, the solidification or adhesion of the aforementioned produced material is likely to occur around the electric component section. In order to avoid the solidification or adhesion of the produced material, conventionally, a heater band 8 is wound at the periphery of a base section 10 as shown in FIG. 7. However, the excessive heating by the heater band 8 may cause the plastic deformation of the impeller 2. In view of this, such a method is adopted in that a water-cooling pipe 12 is closely contacted with the base section 10. In addition, FIG. 7(A) is a longitudinally sectional view (partially schematic view) of the turbo molecular pump in which the heater band and the water-cooling pipe are disposed, and FIG. 7(B) is a sectional view taken along line 7Bxe2x80x947B. As to the heater band 8 and the water-cooling pipe 12, the heater band 8 and an electromagnetic valve 14 determining the flow rate in the water-cooling pipe 12 are ON/OFF controlled so that the detected temperature by a temperature sensor 16 incorporated in the base section 10 becomes a temperature set by a temperature control circuit 18. With this control, the turbo molecular pump has been regarded such that it is possible to prevent the solidification or adhesion of the produced material while maintaining its performance.
However, if the discharged gas flow rate is set large, the conventional control of the heater band 8 and the electromagnetic valve 14 in the water-cooling pipe 12 may sometimes lead to the problem that the temperature of the impeller 2 becomes twice or more of the temperature set by the temperature control circuit 18 by increasing the discharged gas flow rate when the temperature sensor 16 senses the temperature set by the temperature control circuit 18. For example, if the temperature of the base section 10 is tried to be maintained in a range of from 60xc2x0 C. to 80xc2x0 C. under a condition that the discharged gas flow rate should exceed a permissible value, there may arise a case that the temperature of the impeller 2 exceeds its heat-resisting temperature. If this condition lasts for a long time, the stress resistance of the impeller 2 is lowered and the impeller 2 may break. Therefore, in order to avoid the abnormally high temperature of the impeller 2, the power of the motor 30 must be lowered, or the discharged gas flow rate must be restricted.
The cooling of the impeller 2 is mainly relied on the following two phenomena: The first one is for a case where the discharged gas flow rate is small, and of the heat transmission due to radiation from the impeller 2 to the base section 10 or the envelope 11. The second one is for a case where the discharged gas flow rate is large, and of the heat transmission through the discharged gas by the heat transfer from the impeller 2 to the base section 10 or the envelope 11. These radiated and transferred heats are cooled as a consequence of cooling the base section 10 or the envelope 11.
However, if the generated heat of the impeller 2 is large but the discharged gas flow rate is small, for example in case of no load under a magnetic field, then sufficient heat transmission by the radiation cannot be expected, and the temperature of the impeller 2 may become high.
The present invention is made in view of the problems encountered in the conventional art, and an object of the present invention is to provide a turbo molecular pump which can release heat generated in an impeller without cooling a base section.
To attain the above-noted object, according to the present invention, a turbo molecular pump is characterized by comprising: an electric component section including at least a motor; a base section supporting the electric component section; a shaft rotated by the motor; and an impeller fixed to the shaft. The present invention further comprises cooling means for cooling the electric component section, the impeller and the shaft without cooling the base section. The cooling means includes means for cooling only the heat generated in the motor of the electric component section, the impeller and the shaft without cooling the base section in order to prevent solidification or adhesion of the produced material, and more specifically, includes all means by which the generated heat can be absorbed or released through gaseous or liquefied medium.
By this arrangement, it is possible to externally release the heat generated within the electric component section while preventing the solidification and adhesion of the produced material, so that the permissible flow rate or the like can be improved.
Further, the present invention is characterized in that the cooling means is constructed by an arrangement that a bottom cover disposed in a bottom portion of the base section is contacted with a bottom surface of a casing disposed in a bottom portion of the electric component section. The bottom cover is exposed to the external air and thus the heat within the electric component section is subjected to the natural-draft cooling. Further, the heat within the electric component section can be easily radiated.
Further, the present invention is characterized in that a forcible cooling section of water cooling, oil cooling or forcible air cooling is fixed or detachably provided to be contacted with an outside of the bottom cover. By this arrangement, it is possible to easily absorb the heat within the electric component section in an external portion. Since the forcible cooling section is disposed outside the turbo molecular pump, the attachment and detachment thereof can be easily made.
Here, although it may be conceivable that the upper portion or peripheral portion of the electronic component section can be cooled by water cooling, according to the present invention it is also possible to arrange the cooling means such that a casing side cylinder disposed around the electric component section is extended toward a bottom cover disposed in a bottom of the base section, and a forcible cooling section of water cooling, oil cooling or forcibly air cooling is fixed or detachably provided in a space defined by the bottom cover, the casing side cylinder and a bottom surface of a casing disposed in a bottom of the electric component section.
By this arrangement, the structural modification from the conventional turbo molecular pump can be minimized.
Note here that the forcible cooling section may be constructed in various ways as far as it absorbs the heat generated in the electric component section and releases that heat therefrom externally. For example, a water-cooling pipe may be disposed to be contacted with or to be closer to the casing bottom surface, or a formed space may be filled entirely with water. Although the water-cooling pipe should not be limited in shape and length, it is preferable that the water-cooling pipe is configured to provide larger contact area with the casing bottom surface. Further, the medium should not be restricted to water, and any other liquefied or gaseous medium having a large heat exchange effect can be used.
Further, according to the present invention, the cooling means is arranged so that at least one stage purging gas pressure increasing section having at least one inclined groove is provided for increasing pressure in the vicinity of a stator by rotation of the shaft and the impeller in a flow path of purging gas. In the purging gas pressure increasing section, one or more inclined groove(s) is disposed. This inclined groove serves to suppress the discharge of the purging gas as the shaft and the impeller rotate, and thus increases pressure in the vicinity of the stator. The purging gas pressure increasing section can be disposed at any arbitrary portion in the flow path of the purging gas, and it is possible to dispose one or more stage(s) of purging gas pressure increasing section(s) along the flow path of the purging gas. By increasing the pressure in the vicinity of the stator, the heat exchange effect between the impeller and the shaft, and the electric component section can be enhanced.
Further, according to the present invention, the cooling means is arranged to have at least one stage purging gas pressure increasing section having at least one inclined groove for increasing pressure in the vicinity of a stator by rotation of the shaft and the impeller in a flow path of purging gas, and at least one stage processing gas reverse flow preventive section having at least one inclined groove for preventing reverse flow of processing gas. In the flow path of the purging gas, the processing gas reverse flow preventive section is provided in addition to the purging gas pressure increasing section. The processing gas reverse flow preventive section is formed with one or more inclined groove(s). As the shaft and the impeller rotate, the inclined groove(s) serves to prevent the processing gas from flowing reversely. By preventing the processing gas from entering into the electric component section, it is possible to prevent corrosion by the processing gas in the electric component section. The inclined groove may have an inclination required for increasing pressure in the vicinity of the stator and an inclination required for preventing the reverse flow of the processing gas when the impeller rotates. The inclined groove may not be formed without limiting in shape, size, length, number and so on.
Further, according to the present invention, a heat insulating material is partially or entirely interposed in an annular manner between a casing side cylinder disposed around the electric component section and the base section. The heat insulating material thus disposed renders the base section not to be adversely affected by the cooling of the electric component section. Since the base section is not cooled, the radiated heat from the heater band or processing gas never escape to the base section, and therefore it is possible to surely avoid the solidification and adhesion of the produced material.